1. Field of the Invention
The present invention relates to an apparatus for manufacturing semiconductor wafers. More particularly, the present invention relates to an apparatus for chemically and mechanically planarizing a surface of a semiconductor wafer.
2. Description of the Related Art
As semiconductor devices have continued to be scaled down, an interconnection structure thereof trends toward a multi-layer structure. Such a multi-layer structure results in an increase in a surface step difference between unit cells stacked on a semiconductor substrate. To reduce this step difference, various methods for planarizing the surface of a wafer have been proposed. One proposed technique is chemical mechanical polishing (CMP).
In a CMP apparatus, a wafer is typically positioned on a polishing head so that a wafer surface to be polished (i.e., the process surface or polishing surface) faces a polishing pad. Then, the surface of the wafer to be polished is placed on the polishing pad installed on a platen. The polishing head provides a controllable pressing force onto a rear side of wafer to press the wafer against the polishing pad. Simultaneously, the polishing head may be rotated to provide additional movement between the wafer and the polishing pad.
Many manufacturers of CMP apparatus are developing various types of polishing heads having membranes. A polishing head having a membrane chucks a wafer using vacuum pressure and then releases the wafer using forced air.
FIGS. 1A, 1B, and 1C illustrate views of a conventional wafer unloading process. In FIGS. 1A, 1B and 1C, reference numerals 10, 12, 20, and 22 designate a polishing head, a membrane, a transfer stage, and a pedestal, respectively. Reference character W designates a wafer.
Referring first to FIG. 1A, when the polishing head 10 moves to the transfer stage 20, the pedestal 22 of the transfer stage 20 ascends. Then, as shown in FIG. 1B, the membrane 12 is expanded by an air pressure and the wafer W is placed on the pedestal 22. Next, as shown in FIG. 1C, the membrane 12 is contracted after the wafer W has been placed on the pedestal 22.
If the membrane 12 is contracted before the wafer W is completely separated from the membrane 12, for example, when edge portions of the wafer W are separated from the membrane 12 but a center portion of the wafer W still remains in contact with the membrane 12, the wafer W will be lifted up by hanging onto the membrane 12. This lifting will result in a wafer unloading error. Particularly, in a case that a wafer W has a hydrophobic surface, such a wafer unloading error will frequently occur due to the surface tension of a contact surface between the wafer W and the membrane 12.